services: Add 'hurd-vm service-type'.

* gnu/services/virtualization.scm (hurd-vm-shepherd-service,
hurd-vm-disk-image): New procedures.
(%hurd-vm-operating-system, hurd-vm-service-type): New variables.
(<hurd-vm-configuration>): New record type.
* doc/guix.texi (Virtualization Services): Document it.
* gnu/services/shepherd.scm (scm->go): Use let-system, remove FIXME.  Fixes
fixes cross-building of shepherd modules for the Hurd image.